Electrical measuring apparatus, including optical-character-recognition systems, typically employ calibration and adjustment circuits to adjust to variations between sensing elements, in ambient conditions, and among other environmental variables. Typically, the signal for each sensing element is adjusted separately, as the sensing element and the associated circuit elements unique to that element vary significantly in response and characteristics from those of the other sensing elements, even though they are designed to be identical.
Changes in the operative factors may occur continually as a result simply of random variations in the environment, as well as more predictable causes. A major cause of variation is difference in paper characteristic within the same sheet, when printing on the paper is being observed by scanning across the paper.
Depending on the specific problems of a given optical system, adjustment of the sensor with each sensing operation may be desirable. For example, where a white is sensed in an optical system by one sensor and the next sensing is a white significantly more toward white characteristics than the first, adjustment of the sensor to recognize a brighter white as normal may be accomplished. Such continual adjustment as generally conceived forms no part of this invention.
This invention is directed to a specific implementation in which such adjustment is made in a system in which each sensor system includes a current-acceptor such as a capacitor. Adjustment is made by incrementing a count which is the input to an digital-to-analog converter, the output of which controls current into the capacitor.
The use of a count or counter as an input to a digital-to-analog converter is well known in various contexts. Typically, a pulse train has a number of pulses proportional to a number to be digitized. This number may be represent a height of a pulse or the time in a meter. The number of pulses is counted, and the count is the input to the digital-to-analog converter. Documentation representative of this state of the art and also in the field of electrical sensing are U.S. Pat. Nos. 4,172,659 to Laska and 4,136,277 to Gerrard. The capacitive-sensing system employed in the actual embodiments, as described in detail in connection with the first embodiment discussed, is a standard, commercially available scanner and, in itself, forms no part of this invention.